Containers employed in the packaging of foodstuffs and beverages conventionally are comprised of a variety of materials providing a thin-walled resilient construction. Among such materials which can be satisfactorily used are low-carbon steel sheet, commonly referred to as black plate, which can readily be fabricated into container bodies employing conventional cupping and draw and ironing press operations providing economic advantages over aluminum, plastic materials and tin-plated steel sheet. Such drawn and ironed black plate container bodies are characterized as having a desirable light-grey shiny steel surface appearance which provides for an attractive package after subsequent coating with a clear organic lacquer and imprinting ink indicia on the exterior can surface.
During recent years, tests have developed for evaluating the adhesion of the lacquer coating to the surfaces of such black plate containers and the corrosion resistance or resistance to undercutting of the organic lacquer coating adjacent to scratches or imperfections or discontinuities in the lacquer coating. One such test which has received industry acceptance is a so-called "citric acid test" as more fully described in an article entitled "The Undercutting of Organic Lacquers on Steel" by O. D. Gonzalez, P. H. Josephic and R. A. Oriani as published in the Journal of the Electrochemical Society, Volume 121, No. 1, of January, 1974. Briefly stated, the test comprises preparing an aqueous solution containing 15 grams of citric acid and 15 grams of sodium chloride per liter in tap water. Black plate container bodies after cleaning, are dip-coated with a phenolic lacquer to provide a dry coating weight of about 7.2 to 8.8 mg/in.sup.2 and the lacquer coating is air dried for ten minutes followed by curing for ten minutes at about 410.degree. F. The lacquered can bodies are scratched to provide a test area and are immersed in the salt-citric acid solution at room temperature for a period of about four days. At the completion of the test period, the samples are removed, blotted dry and the amount of undercutting of the lacquer from the scratch or score line to the nearest 0.1 millimeter (mm) is measured. The salt-citric acid test provides an agressive environment and there has been a need for an improved treatment of the black plate container body surfaces which increases the adhesion of the lacquer coating to the surface and also resists undercutting of the lacquer coating when subjected to the salt-citric acid test. Such treatment must also provide a surface which retains humidity resistance such as measured during a 16 to 24 hour exposure to 100 percent relative humidity after cleaning and before lacquering while being sheltered from direct condensation in the humidity chamber. The treatment must further not visually detract from the shiny, light-grey metallic appearance of the surface of the container body. Because of this, conversion coatings such as phosphate and chromate coatings are unsatisfactory from an appearance standpoint. Also, chromates are becoming ecologically unacceptable. It is also desirable that such treatment be integrated into conventional container body manufacturing procedures without a disruption of production capacity and control.
The treating process of the present invention attains the aforementioned objectives providing an economical integrated treatment of black plate container bodies achieving increased resistance to lacquer undercutting without loss of humidity resistance and without detrimentally affecting the shiny light-grey metallic appearance of the container surfaces.